Apparatus for melt extrusion of multi-wall plastic tubing



Nov. 28, 1967 G. E. RALEY 3,354,506

APPARATUS FOR MELT EXTRUSION OF MULTI-WALL PLASTIC TUBING Original FiledApril 30, 1962 5 sheetssheet 1 INVENTOR. GARLAND E. RALEY ATTORNEY G. E.RALEY 5 Sheets-Sheet 5 INVENTOR.

ATTORNEY Nov. 28, 1967 APPARATUS FOR MELT EXTRUSION 0F MULTI-WALLPLASTIC TUBING Original Filed April 30, 1962 G. E. RALEY 3,354,506

APPARATUS FOR MELT EXTRUSION OF MULTIWALL PLASTIC TUBING Nov. 28, 1967 5SheetsSheet 4 Original Filed April 30, 1952 R. m m V m GARLAND E. RALEYATTORNEY G. E. RALEY Nov. 28, 1967 APPARATUS FOR MELT EXTRUSION OFMULTI-WALL PLASTIC TUBING 5 Sheets-Sheet 5 Original Filed April 30, 1962INVENTOR. GARLAND E. RALEY K SUBSTRATE E XTRUDER ou-rza COATING INNERCOATING EXTRUDER EXTRUDER ATTORNEY United States Patent Ollice 3,354,506APPARATUS FDR MELT EXTRUSKUN F MULTI-WALL PLASTIC TUBING Garland E.Raley, Morris Plains, N.J., assignor to Union Carbide Corporation, acorporation of New York Original application Apr. 30, 1962, Ser. No.81,230, how Patent No. 3,223,761, dated Dec. 14, 1965. Divided and thisapplication Nov 16, 1965, Ser. No. 508,065

2 Claims. (Ci. 13-14) ABSTRACT OF THE DISCLOSURE This application is adivision of United States patent application Ser. No. 191,238, filedApr. 30, 1962 now United States Patent Number 3,223,761.

This invention relates to apparatus for the simultaneous melt extrusionof a plurality of thermoplastic materials to form tubing having amulti-wall structure.

More particularly, the invention is concerned with certain apparatus forthe production of multi-Wall, tubular thermoplastic film by coaxiallymelt-extruding two or more different thermoplastic compositions intotubing, internally inflating the tubing as it is extruded and while itis yet in the formative plastic state, to a predetermined largerdiameter and then circumferentially chilling the tubing at a point whereit has obtained the desired inflated diameter to cool the tubing to atemperature at which it is resistant to a further expansion by theinternal inflating pressure.

An object of this invention is to provide an apparatus for continuously,coaxially melt-extruding a plurality of different thermoplasticmaterials into integral, multi-wall tubing of uniform, predeterminedthickness in each wall of said tubing.

Another object of this invention is to provide apparatus forconcurrently melt-extruding a plurality of thermoplastic materials intocontinuous multi-wall tubing, each thermoplastic material forming atleast one wall in said tubing.

The term thermoplastic material as used herein is inclusive of allmelt-extrudable thermoplastic polymers and compositions containing suchpolymers in admixture with fillers, pigments, dyes, antioxidants andother additives such as anti-blocking agents, anti-static agents andothers ordinarily employed to modify the characteristics of the polymer.

The several walls of the multi-wall tubing produced according to thisinvention can all contain the same thermoplastic ploymer, but whichwalls differ from each other as for example by the inclusion of acoloring agent in the polymer constituting the inner wall of the tubing,and with the outer wall being clear having been formed by the samepolymer free from any coloring agent. Moreover, the present inventionincludes the production of multi-wall tubing wherein all the walls aremade of polymers of the same genus, but one wall can be of a low averagemolecular weight polymer or a low crystalline content polymer and theother wall of a considerable higher average molecular weight polymer ora high crystalline content polymer depending on the properties desiredin the multi-Wall tubing.

Other and additional objects will become apparent as the descriptioncontinues.

Patented Nova .28, 1967 In general, the objects of this invention areaccomplished by separately heating each thermoplastic material which isultimately to form an individual distinct Wall or layer in the compositetubing to its melt extrusion temperature, as for example by aconventional melt extruder, each extruder separately pressure feeding astream of its melted thermoplastic material into an extended cylindricalchamber, the discharge end portion of said chamber communicating with anannular die orifice, the melted material for forming the outer wall ofthe tubing being fed into said chamber and passed therethrough as acontinuous annular stream contacting the interior Wall of the chamher,the melted materials for forming additional walls between said outerwall and the interior wall of the final tubing, also being fed to saidchamber and passed therethrough as annular coaxial streams concentricwith and interior to said outer wall annular stream, and thethermoplastic material for forming the innermost wall of said tubingbeing pressure fed to said chamber and passed therethrough as a centralcore completely filling the remaining space in said chamber, to form asolid stream of melted plastic materials, the plastic melt flow of saidmaterials through the chamber having Reynolds numbers below thethreshold of turbulence (Nre-ZIOO), discharging the composite,concentric cylindrical solid stream of plastic material centrally intoan annular die assembly having a mandrel axially piercing and radiallydistributing said cylindrical stream into a tubular stream, and thenextruding the pierced stream through the die lips to form continuousseamless tubing.

Surprisingly, it has been found that in the practice of this inventionthe molten masses or" unlike thermoplastic resins or thermoplasticcompositions can be conjoined as annuli enveloping a central cylindricalmass of molten plastic in a chamber which is in spaced relationship toan annular extrusion orifice, and therebetween the conjoined cylindricalplastic mass is advanced, radially distributed into a tubular streamwhich can be attenuated and extruded without changing relativeproportions of the diverse plastic constituents in the cylindricalmolten plastic mass or intermixing of the various plastic materials. Bythis method, for example, a thermoplastic film material having physicalcharacteristics such as a desirable high modulus of elasticity, but pooroptical quality e.g., lacking in clarity, or having an undesirablenarrow heat-seal temperature range can be extruded with a thermoplasticfilm material having a broad heat-seal temperature range, or that hassuperior optical properties to form multi-Wall tubing wherein the outerand/ or inner wall is formed from the broad heat-seal range polymer anda concentric middle lamina in the tubing is formed from the polymerhaving a high modulus of elasticity, The amount of each polymer in agiven wall of the tubing can be exactly proportioned by regulatingindependently the plastic feed delivery of a plurality of extruders tothe chamber wherein the separate streams are coaxialiy distributed andfeeding the conjoined plastic mass to and through the extrusion dieorifice.

An important feature of this invention resulting in the uniform coaxialdistribution into annuli of the diverse plastic materials fed into theelongate chamber, each annulus containing only one plastic material, isthe maintaining of the melt flow or" these materials below the thresholdof turbulent flow whereby substantially laminar flow conditions prevailwithin the chamber and to and through the die orifice.

Another important feature of this invention is the pressure feeding ofthe coaxial cylindrical mass of molten materials whereby each individualannulus in the coaxial melted plastic mass is subjected to continuousheat and pressure until it passes through the die orifice. Inasmuch asall the diverse plastic materials in the coxial mass are in a meltedcondition while concurrently being under u) pressure of the order of 500to 3500 pounds per square inch or higher, developed by the continuousfeeding to the chamber of melted plastic from the several extruders,optimum bonding conditions are present for securing tenacious bondingtogether of each annulus of plastic material to its adjacent annulus ofdifferent plastic material.

It is to be noted that the exposure to heat and pressure of the diverseplastic materials in the molten coaxial stream is maintained for aconsiderable period of time as the coaxial stream slowly flows towardsthe die orifice. In other procedures heretofore proposed forcontinuously heat and pressure laminating together films of dissimilarplastic materials, it has not been commercially feasible tosimultaneously heat all the films to a molten condition, and stillmaintain their physical dimensions and not fiow unduly when subjected tobonding pressure. In general, such previously proposed proceduresdepended upon heating one of the plastic films to an elevatedtemperature but below that at which it was no longer selfsupporting andpassing the heated film and unheated different film between counterrotating rollers, the two films being only momentarily subjected topressure as they passed through the nip of the rollers, the resultantlaminate having generally a poor bond strength between the adjacentfilms.

In the accompanying drawings, there is shown several embodiments ofextrusion apparatus effective for forming multi-wall thermoplastictubing according to this invention and wherein:

FIGURE 1 is a diagrammatic side elevation of an extrusion apparatus forforming coaxially melt-extruded, multi-wall thermoplastic tubular film.

FIGURE 2 is a diagrammatic fragmentary section of the apparatusgenerally shown in FIGURE 1 and illustrating the cylindrical chamber inwhich the plastic stream from each extruder is molded into coaxialstreams.

FIGURE 3 is a sectional view taken along the line 3-3 of FIGURE 2.

FIGURE 4 is a sectional view 4-4 of FIGURE 2.

FIGURE 5 is a sectional view 5-5 of FIGURE 2.

FIGURE 6 is a sectional view 6-6 of FIGURE 2.

FIGURE 7 is a sectional view 7-7 of FIGURE 2.

FIGURE 8 is a sectional view 8-8 of FIGURE 2.

FIGURE 9 is a sectional view 9-9 of FIGURE 2.

FIGURE 10 is a sectional view taken along the line 10-10 of FIGURE 2.

FIGURE 11 is a vertical sectional view of an annulus forming chamber.

FIGURE 12 is a sectional View taken along line 12-12 of FIGURE 11.

FIGURE 13 is a somewhat enlarged perspective view, partially brokenaway, of the plastic flow control metering valve and coaxial streamforming means shown in FIGURE 2, and,

FIGURE 14 is a diagrammatic side elevation of a melt extruding apparatusfor forming a triple wall tubular film.

Referring now to FIGURES l to 13 wherein like reference numeralsdesignate like parts, a primary thermoplastic resin extruder generallyindicated by the reference numeral 10 is used to feed a melt flow 16 ofthermoplastic resin that ultimately constitutes the middle layer orsubstrate of a triple wall tubular film structure 30. Extruder 10 has afeed hopper 12 for receiving thermoplastic resin pellets, a screwchamber 13 and screw 14 for compacting, melting, mixing and advancingthe resin to and through a perforate breaker plate 18. The extruder It)has a jacketed chamber 19 supplied with heated oil or other suitablefluid for heating the thermoplastic resin as taken along the line takenalong the line taken along the line taken along the line taken along theline taken along the line it is advanced from hopper 12 to the dischargeconduit of the extruder.

A secondary thermoplastic resin extruder generally indicated by thereference numeral 20 is used to feed a melt flow 25 of thermoplasticresin that constitutes the supply for an inner coaxial wall forming flow26 and an outer coaxial wall forming flow 27 of the laminated triplewall tubular film structure 30. Extruder 20 has a feed hopper 22 forreceiving thermoplastic resin pellets, a screw chamber 23 and screw 24for compacting, melting, mixing and advancing the melted resin to andthrough a perforate breaker plate 28. The extruder 20 has a jacketedchamber 29 supplied with heated oil for melting the resin supply as itis advanced from hopper 22 to the discharge conduit of the extruder.Each extruder has a gauge P for measuring the pressure of the meltedplastic and a thermometer T for measuring the temperature of theplastic.

The plastic melt flow from extruder 10 is discharged therefrom throughits tapered discharge conduit to an elbow 32 connected to a port oftriple ported T fitting 34, another port being connected to thedischarge end of extruder 20 and the third port being connected to avertical die holder 36 having a cylindrical central passage 78therethrough.

A multi-ported torpedo assembly for channeling the flow of meltedplastic from extruder 10 into an annular stream 16, and of meltedplastic from extruder 20 conjointly into an annular stream 27 exteriorlyencircling annular stream 16 and a solid circular stream 26 interiorlyof annular stream 16 is positioned within the lower portion of centralpassage 78 in die holder 36 and extends downwardly through T fitting 34.

Torpedo assembly 75 as more particularly shown in FIGURES 2 to 9, and 11to 13 comprises a cylindrical metal tube 80 having its lower endthreadably engaged in the bottom port of T fitting 34 and a flange 83mating with the flanged section of elbow 32 on extruder 10. A lock nut82 secures tube 80 to T fitting 34.

Cylindrical tube 80 has, an integrally attached extensions to its outersurface and adjacent its upper end, three equidistant spaced streamlinedspiders 81 to center the tube in passage 78.

Concentrically positioned within tube 80 and fastened thereto as bywelding, is an inner tube 84 having an extensions to its outer surface,and adjacent its upper end, three equidistant spaced streamlined spiders85 to center tube 84 interiorly to tube 80, and has on its lower end aproportioning chamber. Tube 84 has a central cylindrical passage 88 anda communicating transverse passage 90 and is press fitted interiorlyinto assembly with tube 80 so that passage 90 of the proportioningchamber is aligned with passage 92 of tube 80 and also with centralpassage 76 of T fitting 34. The molten plastic 25 flows from secondaryextruder 20 toward the proportioning chamber and is divided at theentrance to cojoined passages 92 and 90 thereof, a portion of the moltenplastic coating flow rising as an annulus in the passage 93 formed bythe outer surface of tube 80 and the inner wall of passage 78, and thebalance of the flow rising as a cylinder in passage 88. An adjustmentvalve stem 86 is threadably fastened into the outboard end of T fitting34 and mounted central to passage 90. The threaded adjustment advancesor retracts the rounded end 87 of valve stem 86 and by restricting orenlarging the effective opening of passage 90 thereby proportions therelative amount of solid circular plastic stream flow 26 with respect toouter wall-forming plastic flow 27 passing upwardly through annularpassage 93.

The proportioning chamber at the lower end of tube 84 has its lower endouter walls flattened on two sides parallel to each other and to passage90 and its lower extremity streamlined to form a truncated wedge whichfibrucates and guides the plastic fed thereto from extruder 10 about theproportioning chamber. Referring to FIGURES 4, 5, 6 and 12, the passage95 formed by the flattened outer walls of 84 and the inner wall of tube86 guides the two streams 16', 16" about the proportioning chamber andreunites them in combined flow rising as an annulus in the upperextension of passage 95 formed by the outer surface of tube 84 and theinner surface of tube 80.

As will be noted by reference to FIGURE 3 which is a section adjacentflange 83, the plastic melt flow 16 is in the form of a solid streamprior to contacting the flattened lower end walls of tube 84. Uponcontacting the flattened end wall the plastic melt flow 16 as shown inFIGURE 4 is bifurcated into two streams 16' and 16".

As shown in FIGURES 5 and 6, streams 16 and 16" are maintained separatefrom individual plastic melt flows 26 and 27 as they flow upwardly andby the proportioning valve means 87, 90.

As shown by FIGURE 7, the coaxial plastic melt streams 27 and 16 areeach parted into three sectors by spiders 81 and 85 respectively, whileplastic melt stream 26 continues its flow as a solid cylindrical stream.Upon emerging from the spider section of torpedo assembly 75, the threesectors of melt stream 27 again unite as the outermost coaxial stream,the three sectors of melt stream 16 also unite as the middle coaxialstream and solid melt stream 26 forms the inner core of a compositecoaxial solid stream as shown in cross-section by FIGURE 8. This solidstream is maintained under intense hydraulic pressure as it is movedupwardly in cylindrical passage 78 to an annular die assembly generallyindicated at 38, and thereby produces a strong bonding effect betweenthe interfaces of the several coaxial streams.

The annular die assembly comprises a conical die cup 41 and a dependingcone 40 spaced therefrom to form a tapered annular passage 37 forsupplying thermoplastic material from cylindrical passage 78 to anannular die orifice 42. The bottom portion of die assembly 38 has aflanged base mating with a flanged end portion of die holder 36, and isfastened thereto by bolts not shown. Although a tapered passage 37 isillustrated in the drawings, a passage formed between parallel walls canbe used successfully. Moreover, the depending cone need not terminate ina sharp tip but can be made as a right truncated cone having a planecircular face. Alternatively, there can be used a right cylindrical corepiece instead of the cone 44 The composite solid stream of coaxiallydistributed melted plastic materials as shown in FIGURE 8 is radiallypierced by the tip of cone 4t) and is diverged outwardly thereby intopassage 37, still maintaining, however, the sharp interfacial coaxialrelationship of the several plastic streams as shown in FIGURE 9 andwhich continues as the conjoint melted streams are extruded throughannular die orifice 42 to form the triple-wall tubing 30 whosecross-section is shown by FIGURE 10.

The tubing 30 emerges from the die orifice 42 in a not formative plasticstate enabling it to be radially distended and its Wall thicknessattenuated by the inflating pressure of a gaseous medium such as air orother suitable gas introduced into the interior thereof from an orifice44 in cone 40. Air under controlled pressure is supplied to orifice 44by its connection to a supply line 46.

The inflated tubing St) is drawn upwardly by a pair of drivencounter-rotating squeeze rolls 48 and 5t) disposed in spacedrelationship to the die. The tubing 30, as it is drawn from the die,passes interiorly through a rotating ring 52 which applies a coolinggaseous medium in the form of an annular stream onto the peripheral wallof the tubing to chill the plastic to a set condition. The rotating ring52 is rotatably mounted on annular cylindrical housing 54 to which thegaseous cooling medium is supplied by a plurality of blowers 56 (onlyone of which is shown). Rotating ring 54 is formed with a pulley surfacethat cooperates with continuous belt 58 driven by a speed reducer-motorcombination unit generally indicated by 60. The die 38 and air ringhousing 54 are mounted on cross-members 62 of the machine framegenerally indicated by 64. The rotating air ring herein described is ofthe type described and contemplated by R. F. Pierce in US. Patent No.2,632,206.

Squeeze roll 48 has a shaft extension at each end to mount rotatably inbearings 66 mounted on frame crossmembers 68; and squeeze roll 50 has ashaft extension at each end to mount rotatably in bearings 70 alsomounted on frame cross-members 68.

The rolls 43 and 50 in addition to confining the inflating air in thetubing 30 also serve to collapse the tubing passing therebetween into aflattened, ribbon-like material, designated by the reference number 72which is wound up on a windup reel driven by a torque motor, not shown,and guided thereto by guide rolls 74.

In the embodiment of the invention for coaxial extrusion ofthermoplastic tubing by the multiple extruder apparatus illustrated inFIGURES l to 13, it is economically desirable to employ a startupprocedure that quickly stabilizes continuous production of the tubing ofpredetermined diameter and wall thickness and wherein theconstituentlamina or individual wall thicknesses are proportioned to apredetermined ratio. As a general rule, the first plastic material to bemelted, pressurized and fed therethrough to the die orifice 42 should bethat material forming the outer wall of the composite tubing.Thereafter, upon attaining a steady flow of this plastic material, theextruder supplying desired plastic material for the inner and outerwalls is operated to supply melted material to the die orifice. Foroptimum efliciency in the startup of the apparatus of FIGURES 1 to 13 toextrude triple wall tubular film having a middle wall of one materialand inner and outer walls of a different plastic material than themiddle wall, the following steps are taken:

(A) Both extruders 10, 20 are placed in operation with valve 87 being inan open position, and each extruder supplying during this interim thatmelted resin which subsequently forms the interior and exterior walls oftubing 39 resulting in tubing having the same resin throughout itsthickness.

(B) The screw of extruder 20 is temporarily stopped and thus ceases tofeed melted plastic material to the die orifice.

(C) Throughput (at the die orifice) from extruder 10 discharging meltedresin, is adjusted to produce tubular film having a wall thicknesscorresponding to that desired in the middle film wall 16 of tubing 30.

(D) With extruder l0 continuing to operate under the conditionsestablished in step C, extruder 20 is restarted and its feed rateadjusted to supply an amount of resin such that the resultant film hasthe total wall thickness desired in tubing 30. During this step, valve87 is so adjusted to furnish a desired thickness in the outside wall 27and a desired thickness in the inner wall 26.

(E) Upon attaining desired wall thickness, the resin supply for extruder10 is changed to a resin having the particular properties desired formiddle wall 16, thereby purging the passage system connected to extruder10 of the resin first extruded therefrom and establishing an annulus ofthe desired middle wall resin in the stream of melted plastic materialsadvancing to the die orifice.

In general, the walls of the die mandrel 40 are more readily purged ofan undesired resin melt than are the walls of the die cup 41 andaccordingly the foregoing procedure facilitates rapid purging of theextrusion system whereby each wall of the triple wall tubing isconstituted solely of desired plastic material.

The details and manner of practicing the invention are apparent in thefollowing specific examples, it being understood, however, that they aremerely illustrative embodiments of the invention and that the scope ofthe invention is not restricted thereto.

It will be noted that in all of the subsequent examples the apparatuswhich was actually used to form the multi-wall thermoplastic tubingcorresponded essentially 7 to that shown in FIGURES 1 to 13 of thedrawing. As will be described in these examples, the apparatus can bereadily operated to form a dual wall plastic tubing as in Example 1,triple wall tubing as in Examples II, III, IV and V, or single walltubings as in the instance for the controls of Examples III and IV.

Example I A dual wall self-supporting tubular polyethylene filmaveraging 3.5 mils in thickness, one wall being a black pigmentedpolyethylene and the other wall a white pigmented polyethylene, wasprepared by melt-extruding a black polyethylene composition comprising3.5 parts by weight of carbon black and 96.5 parts by weight of a filmgrade polyethylene having a melt index value of 2.0 (ATSM testD-l23852T) and a density of 0.92 through extruder 10 and concurrentlymelt-extruding from extruder 20 a white polyethylene compositioncomprising 5.0 parts titanium dioxide and 95 parts of the samepolyethylene as in the black composition. During the extrusion,proportioning valve 87 was closed to prevent any fiow of the whitepolyethylene composition through passage 88. The extrusion conditions ofpressure, temperature and die orifice 42 dimensions for this example andall the others are set forth in subsequent Table I which also includesdata on the physical properties of the multiwall extruded film. Theresultant dual wall tubular film was opaque, the interior wall beingwholly black in color and the exterior wall being completely white withno visible strike-through of black material. The tubular film slittedinto flat sheeting had utility due to its opaqueness as aself-supporting wrapping material for photographic film, and as windowshade material, the white side being aesthetically desirable and theother black side desirable for room-darkening purposes. No delaminationof the two layers occurred when the film was repeatedly flexed.

Example II A triple wall tubular self-supporting fil-rn was formed bymelt-extruding from extruder 10, a film grade polyethylene having a meltindex of 5 and a density of 0.960 to form the middle wall of the tubing,while concurrently melt-extruding from extruder 20 a film gradepolyethylene having a melt index of 2.3 and a density of 0.923 to formthe inner wall and outer wall of the tubing. In this instance, theproportioning valve 87 was fully opened to provide an inner wallthickness equal to the outer wall thickness. The resultant triple walltubing was useful as a heat-scalable packaging film. It had morestiffness than a film of the same thickness made wholly from the 0.923density polyethylene, such stiffness being desirable for use inpackaging machinery requiring a stiff material for consistent feedingand manipulation therein. Additionally, inasmuch as each side of thefilm was provided with a surface of the lower density polyethylene itcould be heatsealed satisfactorily over a broader range of sealingtemperatures, pressures and contact times than film made wholly from thehigher density polyethylene.

Example III Polyethylene in the higher density range, e.g., 0.935 andhigher generally yield, self-supporting films having poor opticalproperties such as greater haze than films of lower density polyethylenewhen extruded by the blown tube, air-cooled process such as is describedby Fuller in US. Patent No. 2,632,206. It has now been found that theoptical properties of such high density polyethylene can besubstantially improved by sandwiching such polymers as the middle wallin a triple wall tubing between an outer wall and an inner wall, each ofa polyethylene having a density of up to about 0.925. A triple wall,self-supporting, clear polyethylene film was continuously produced bymelt-extruding from extruder 10, a film grade polyethylene having a meltindex of 1.5 and a density of 0.937 to form the middle wall of thetubing and concurrently melt-extruding from extruder 20 a film gradepolyethylene having a density of 0.923 and a melt index of 2.3. Theseveral walls of the resultant triple wall film were firmly bondedtogether and did not delaminate when repeatedly manually flexed.Moreover, the triple wall film had a higher gloss and clarity than a.single wall film of the same total thickness prepared by melt extrudingtubular film from the apparatus using only extruder 10 to supply thepolyethylene of 0.937 density. The triple wall film was useful as aheat-scalable packaging film for wrapping bread and other food stuffs.

Example IV Self-supporting film grade polypropylenes, as do the higherdensity polyethylenes, yield tubular film of poor optical propertieswhen made by the blown tube process described by Fuller in US. Patent2,461,975. Moreover, polypropylene films are more difficult to heat-sealdue to their relatively sharp melting points. According to the processof this invention, these deficiencies are substantially overcome bysandwiching the polypropylene as a middle wall between an inner wall andouter wall of polyethylene having a density of up to about 0.925 in atriple Wall tubular film construction. Thus, a polypropylene-containingtriple wall tubular film of improved clarity and heat-scalability wasproduced by meltextruding from extruder 10, a film grade polypropylenehaving a density of 0.91 and a melt index of 5.0 and concurrently meltextruding from extruder 20 a film grade polyethylene having a melt indexof 2.3 and a density of 0.923 to form the inner and outer walls of theresultant triple wall tubing. This tubing had significantly betteroptical properties than a control melt-extruded tubular film ofidentical total film wall thickness containing only the aforedescribedpolypropylene resin. The triple wall tubing was useful as a packagingfilm for bread and other food stuffs.

Example V Self-supporting items of polyamides such as nylon-6 are usefulas packaging films requiring greaseproofness and abrasion resistance.Such films composed solely of nylon-6 polymer are presently more costlythan polyethylene films which, however, have less resistance to greaseand fats. It has now been found that such polymers can be advantageouslycombined as in a dual wall tubular film wherein one of the walls iscomposed solely of nylon-6 polymer and other wall of polyethylene.Typically, a dual wall tubing was prepared by melt-extruding fromextruder 20 a film grade polyethylene having a density of 0.920 to formthe outer wall. In this instance proportioning valve means 87, 90 wasclosed. The resultant dual wall tubular film was useful as a packagingmaterial in applications Where the fatty material or grease was indirect contact with the nylon-6 wall surface. The bond strength of thenylon wall to the polyethylene wall was such that after limited manualflexing of the film, the polyethylene film could be peeled ofif thenylon surface.

It is to be understood that the invention is not restricted to theploymers specifically exemplified in the examples supra. In general, theinvention can be utilized with any melt-extrudable thermoplsaticmaterial, including mixtures of such thermoplastics and syntheticrubbers. Illustrative of other suitable thermoplastic polymers arepolystyrene, copolymers of styrene with other polymerizable monomers asfor example styreneacrylonitrile copolymers, fluorocarbon polymers suchas polytrifluorochlorethylene, polycarbonates, polyethylene oxides,polyvinyl alcohol, cellulose acetate, cellulose butyrate, cellulosepropionate, acrylate and methacrylate homopalymers and copolymers,linear polyesters as for example polyethylene terephthalate, polyvinylchloride rigid or plasticized, copolymers of vinyl chloride as forexample vinyl chloride-vinyl acetate copolymers and vinylchloride-vinylidene chloride copolymers. Moreover, the properties ofthese thermoplastic polymers, as is well known, can be modified by theincorporation therein of suitable modifying agents, such asplasticizers, fillers, coloring agents, antioxidants, heat and lightstabilizers, destaticizers and other.

In the several examples, the invention was exemplified by the productionof multi-wall tubular film of varying total wall thickness. Such tubingcan be made by this invention to a total wall thickness as low as0.0004. Maximum Wall thickness is primarily determined by the end userequirements of the tubing and hence can, if desired,

be of the order 0.030 inch, 0.050 inch, 0.10 inch or even higher.Expansion of the tubing by inflating internal air pressure is adesirable technique for producing thin-wall tubular film, however, theextruded tubing need not necessarily be expanded in those instancesWhere the as is extruded wall thickness is satisfactory for the intendeduse of the tubing.

The degree of bond strength between the several Walls of a unulti-Walltubing produced according to this invention, is primarily dependent onthe degree of compatibility or incompatibility of the polymer in a givenwall with respect to the polymer in an immediately adjacent wall. Forexample, the highest values in bond strength are obtained in wallcombinations of similar polymers as for example between a wall of lowdensity polyethylene and a wall of medium density polyethylene. A lesserdegree of bond strength is exhibited between a wall of polyethylene anda wall of polypropylene. Radically different polymers have generally thelowest bond strength to each other as for example a imulti-wallstructure of polyethylene and nylon-6 as shown in Example 5. Anotherexample of still lower bond strength is that of a multiwall tubular filmin which a wall of polyethylene is adjacent to a wall of polyethyleneterephthalate, such tubing can usually be delaminated if desired byapplying a peeling force insufiicient to tear the individual walls,whereas in the instance of nylon and polyethylene, the somewhat higherdegree of adhesion can cause tearing of one or both walls during peelingof one Wall off the other Wall.

if desired. For example, a triple wall extruded tubular structure havinga different polymer in each wall can be readily produced by theapparatus illustrated in FIG- URE 14, wherein each of the threeextruders is supplied with a different polymer for coaxial feeding intoa single common passage terminating in an annular single cavity dieassembly. If more than three walls are desired, the apparatusillustrated in FIGURE 14 can be modified by the addition of the requirednumber of extruders to supply melted plasticmaterial for the additionalwalls.

As it is obvious that various changes and modifications may be made inthe above description without departing from the nature or spiritthereof, this invention is not restricted thereto, except as set forthin the appended claims.

What is claimed is:

1. Apparatus for melt-extruding multi-wall thermoplastic tubingcomprising a single cavity annular die assembly, a body having acylindrical chamber therein with one end of said chamber terminating atthe entrant portion of said die assembly, a plurality of extruders, apassage means between each extruder and said cylindrical chamber forpassage of melted plastic material from the extruder to the chamber,each passage means discharging plastic material into the chamber at apoint spaced from the chambers die terminating end, means in saidchamber for forming therein from all the melted plastic materialssupplied thereto a central cylindrical core of one plastic materialabout which are coaxially formed cylinders of other plastic materials toform a solid laminate stream of said plastic materials and meansassociated with said die assembly for axially piercing and radiallydistributing the core containing solid stream into a tubular stream.

2. Apparatus for melt-extruding multi-Wall thermoplastic tubingcomprising a single cavity annular die assembly, a body having acylindrical chamber therein with one end of said chamber terminating atthe entrant portion of the die assembly, a first extruder and a secondex- TABLE I Example N o I II III V) IV V Extrudcr (10):

Barrel Dia.., in

Plastic Melt Temp, F. Plastic Melt Press, p.s.i

Extruder (20):

Barrel Dia., inc.

(30): Diameter, Wall Thickness (mils):

Inner Wall (26) Middle Wall Outer Wall (27) Physical Properties:

Haze (A.S.T.l\I.D-1003 59'l) Gloss: Gardner Photometer) Modulus at 1%Elongation:

(1X10 p.s.i.) MD

(A.S.T.M.D15305ST) TD The number of walls in a given multi-wall tubularstructure and the nature of the polymer in each is determinedessentially by the use properties desired in the structure. Each wallcan be formed of a different polymer,

truder, a passage means between each extruder and said cylindricalchamber for flow of melted plastic from its extruder to the chamber,each passage means discharging plastic material into the chamber at apoint spaced from 1 1 the chambers die entrant end, a torpedo positionedin said chamber having a central passageway therethrough one end of saidpassageway connecting with the passage means from the first extruder andthe other end being positioned to discharge a central core of plasticmaterial in said chamber, a first annular passageway in said torpedo oneend thereof being connected to the passage means of the first extruderand other end being positioned to discharge an annular stream of meltedplastic material contacting the inner chamber wall, a second annularpassageway in said torpedo one end thereof being connected to thepassage means of the second extruder and other end being positioned todischarge into the chamber a second annular stream of melted plasticmaterial between the first annular stream and the core stream and formtherewith a solid stream, and means associated with said die assemblyfor axially piercing and radially distributing the core containing solidstream into a tubular stream.

References Cited UNITED STATES PATENTS 10/1934 Megow.

9/ 1943 Radford.

11/ 1951 Clinefelter 18-13 XR 4/1952 Colombo 18-14 3/1953 Pierce 18--14XR 9/ 1955 Beare.

FOREIGN PATENTS 7/ 1955 France. 11/1961 Germany.

WILLIAM J. STEPHENSON, Primary Examiner.

1. APPARATUS FOR MELT-EXTRUDING MULTI-WALL THERMOPLASTIC TUBINGCOMPRISING OF SINGLE CAVITY ANNULAR DIE ASSEMBLY, A BODY HAVING ACYLINDRICAL CHAMBER THEREIN WITH ONE END OF SAID CHAMBER TERMINATING ATTHE ENTRANT PORTION OF SAID DIE ASSEMBLY, A PLURALITY OF EXTRUDERS, APASSAGE MEANS BETWEEN EACH EXTRUDER AND SAID CYLINDRICAL CHAMBER FORPASSAGE OF MELTED PLASTIC MATERIAL FROM THE EXTRUDER OF THE CHAMBER,EACH PASSAGE MEANS DISCHARGING PLASTIC MATERIAL INTO THE CHAMBER AT APOINT SPACED FROM THE CHAMBER''S DIE TERMINATING END, MEANS IN SAIDCHAMBER FOR FORMING THEREIN FROM ALL THE MELTED PLASTIC MATERIALSUPPLIED THERETO A CENTRAL CYLINDRICAL CORE OF ONE PLASTIC MATERIALABOUT WHICH ARE COAXIALLY FORMED CYLINDERS OF OTHER PLASTIC MATERIALS TOFORM A SOLID LAMINATE STREAM OF SAID PLASTIC MATERIALS AND MEANSASSOCIATED WITH SAID DIE ASSEMBLY FOR AXIALLY PIERCING AND RADIALLYDISTRIBUTING THE CORE CONTAINING SOLID STREAM INTO A TUBULAR STREAM.